For some reason, linking with libvw doesn't work on osx. Instead, libCore libMath, etc must be individually linked.
I'm not sure if this is related, but on linux I find that an ldd of libvw outputs:
libvwCore-2.1.0_post.so => not found
libvwMath-2.1.0_post.so => not found
libvwImage-2.1.0_post.so => not found
libvwFileIO-2.1.0_post.so => not found
This is probably a user configuration issue.